The internal combustion engine has undergone very little change since the end of the Second World War and it is arguable that the piston engine in almost universal use today has more in common with design practice at the outbreak of WW2 than the developments that took place during it.
With the war over, the requirement was for something which was cheap and simple to produce, something that did not require especially advanced machining techniques or exotic materials. At the time, the Internal Combustion Engine was seen as nothing more than a stop gap until another technology took over, and this assumption has been repeated time and time again over the intervening decades.
The rotary engine was one of a number of post-war developments. The basic geometry of the rotary engine had been around since the age of steam, but the work done by Felix Wankel brought it to the point where it seemed to offer a viable solution. Compared to a conventional Internal Combustion Engine the Wankel was much smaller for a given capacity and offered the added bonus of near vibration-free running. By the end of the decade almost every major engine producer had bought licences from the Wankel Institute to produce rotary engines, but problems soon became apparent. The first, poor sealing of the rotor tips, was eventually overcome, although only after many product recalls and warranty claims had brought pioneering manufacturers near to bankruptcy. The second was inherent in the basic design. The geometry of the spinning rotor and the chamber wall in a Wankel force the combustion volume to be whatever is left over between the rotor and the chamber wall at the point of combustion. This leads to incomplete combustion due to the large area/volume ratio and inefficient shape. Poor fuel consumption was thought something the world could live with until the arrival of the first fuel crisis in 1974 but almost overnight, the Wankel was abandoned by all the major players except Mazda. With emissions as important today as fuel consumption, the inefficient combustion of the Wankel makes it hard to see it play any significant role in the future except in applications where the user is able to tolerate these deficiencies in exchange for its small size and smoothness.
Their fingers burned, manufacturers returned to conventional engines and waited for advances in what was accepted to be the power source of the future; the fuel cell. Developments in advanced batteries are also ongoing but it ironic that at the very time the first commercial electric vehicle can be realistically contemplated, there is a growing realisation that the Internal Combustion Engine will have a significant future for many years as a highly efficient unit on its own or as part of a hybrid powertrain.
In some respects, the designers of diesel engines for commercial use have been more adventurous with opposed piston 2-strokes being developed in Germany in the 1930s with the Junkers Jumo and in the 1950s and 60s with the Napier Deltic and the Commer TS3.
Having established the requirement for a modern piston engine, the question is what characteristics should it have?
For reasons of manufacturing efficiency and reliability in use, the engine should be as simple as possible with the minimum of moving parts. It should be as small as possible, so that it can be easily accommodated in whatever it is powering, improving the packaging of a vehicle, enhancing pedestrian safety and making its incorporation into a hybrid powertrain simpler.
It should be lightweight, so that minimum energy is used in moving its own mass, while a larger payload is made possible within a fixed gross vehicle weight.
It should use its fuel more efficiently than contemporary engines, meaning better combustion, less internal friction and reduced reciprocating masses.
If possible, it should be vibration free to improve the comfort of vehicle occupants and reduce the stresses in the chassis.
Theoretically, a 2-stroke engine has an advantage in achieving a greater output for its size, since each cylinder produces power every revolution. However, this potential has been compromised by excessive inlet and exhaust period overlap and the fact that piston-opposed 2-strokes have not traditionally had optimum asymmetry to achieve the most efficient combustion. Advances in fuels, injection systems and engine management mean that much of this can be overcome, with a cleaner engine emerging.
A 2-stroke internal combustion engine with opposed cylinders, each cylinder having a pair of opposed pistons connected to a common central crankshaft, has been disclosed by Hofbauer in U.S. Pat. No. 6,170,443. Independent angular positioning of the eccentrics on the crankshaft allows for asymmetrical timing of the intake and exhaust ports, thus optimising the inlet and exhaust port overlap. The effect of the resulting primary dynamic imbalance is minimised by controlling the geometrical configurations and masses of the moving parts in a complex manner.
Another contributor to the emissions produced and overall inefficiency of a conventional engine is that brought about by piston side thrust caused by connecting rod/crankshaft geometry, while frictional losses due to combustion forces acting directly on the big-end and main bearings are significant.
Improvements in all of these areas can only be beneficial.